This invention relates to compression machinery, and in particular, to a method and associated apparatus to accomplish stable starting and shutdown of multi-stage compression equipment employing a single aftercooler.
In multiple stage compression machinery of the type wherein each compression stage is sequentially placed in operation only after the next lower stage has been operating at design conditions for a predetermined period of time, discharge gas coolers or aftercoolers are required to reduce the temperature of fluid discharged from each stage. As is well recognized, a substantial reduction or elimination of the heat developed by compression of the fluid is particularly important when the fluid is recirculated from the discharge side to the suction side of the compression stage. Heretofore, it has commonly been the practice to have separate gas coolers for each separate compression stage.
In starting multi-stage compression machinery, the lowest pressure stage is initially activated. The fluid discharged therefrom is directed through a bypass or recirculation path including the aftercooler. The low pressure stage is operated in this manner for a predetermined time interval to insure that all mechanical parts of the equipment are functioning properly and to further permit the unit to thermally expand at minimal load conditions.
When the next higher pressure stage is started, the fluid discharged therefrom is directed through a bypass or recirculation path also including the gas cooler. A portion of the fluid directed through the low pressure stage bypass path is now directed to the suction side of the operating high pressure stage. As flow requirements of the high pressure stage are increased, an increased proportion of the fluid discharged from the low pressure stage is diverted to the suction side of the high pressure stage. The increased flow to the high pressure stage and concurrent decreased flow through the low pressure stage bypass path should be accomplished in an efficient manner to avoid a loss of operating efficiency and to prevent the creation of operating problems, as for example surge conditions. Additionally, it is important that in installations having a number of multi-stage machines operating concurrently to handle a single load, that either of the stages of a single machine can be independently stopped without interfering with the operation of the remaining stages.